The IPv4 layer generates an IP header when sending a packet unless the IP_HDRINCL socket option is enabled on the socket. When it is enabled, the packet must contain an IP header. For receiving the IP header is always included in the packet.
Only processes with an effective user ID of 0 or the CAP_NET_RAW capability are allowed to open raw sockets.
All packets or errors matching the protocol number specified for the raw socket are passed to this socket. For a list of the allowed protocols see RFC 1700 assigned numbers and getprotobyname(3).
A protocol of IPPROTO_RAW implies enabled IP_HDRINCL and is able to send any IP protocol that is specified in the passed header. Receiving of all IP protocols via IPPROTO_RAW is not possible using raw sockets.
|IP Header fields modified on sending by IP_HDRINCL|
|IP Checksum||Always filled in.|
|Source Address||Filled in when zero.|
|Packet Id||Filled in when zero.|
|Total Length||Always filled in.|
If IP_HDRINCL is specified and the IP header has a non-zero destination address then the destination address of the socket is used to route the packet. When MSG_DONTROUTE is specified the destination address should refer to a local interface, otherwise a routing table lookup is done anyway but gatewayed routes are ignored.
If IP_HDRINCL isn't set then IP header options can be set on raw sockets with setsockopt(2); see ip(7) for more information.
In Linux 2.2 all IP header fields and options can be set using IP socket options. This means raw sockets are usually only needed for new protocols or protocols with no user interface (like ICMP).
When a packet is received, it is passed to any raw sockets which have been bound to its protocol before it is passed to other protocol handlers (e.g. kernel protocol modules).
Raw sockets use the standard sockaddr_in address structure defined in ip(7). The sin_port field could be used to specify the IP protocol number, but it is ignored for sending in Linux 2.2 and should be always set to 0 (see BUGS) For incoming packets sin_port is set to the protocol of the packet. See the <netinet/in.h> include file for valid IP protocols.
In addition all ip(7) IPPROTO_IP socket options valid for datagram sockets are supported.
A raw socket can be bound to a specific local address using the bind(2) call. If it isn't bound all packets with the specified IP protocol are received. In addition a RAW socket can be bound to a specific network device using SO_BINDTODEVICE; see socket(7).
An IPPROTO_RAW socket is send only. If you really want to receive all IP packets use a packet(7) socket with the ETH_P_IP protocol. Note that packet sockets don't reassemble IP fragments, unlike raw sockets.
If you want to receive all ICMP packets for a datagram socket it is often better to use IP_RECVERR on that particular socket; see ip(7).
Raw sockets may tap all IP protocols in Linux, even protocols like ICMP or TCP which have a protocol module in the kernel. In this case the packets are passed to both the kernel module and the raw socket(s). This should not be relied upon in portable programs, many other BSD socket implementation have limitations here.
Linux never changes headers passed from the user (except for filling in some zeroed fields as described for IP_HDRINCL). This differs from many other implementations of raw sockets.
RAW sockets are generally rather unportable and should be avoided in programs intended to be portable.
Sending on raw sockets should take the IP protocol from sin_port; this ability was lost in Linux 2.2. The workaround is to use IP_HDRINCL.
Linux 2.0 enabled some bug-to-bug compatibility with BSD in the raw socket code when the SO_BSDCOMPAT socket option was set --- since Linux 2.2, this option no longer has that effect.
When the IP_HDRINCL option is set datagrams will not be fragmented and are limited to the interface MTU.
Setting the IP protocol for sending in sin_port got lost in Linux 2.2. The protocol that the socket was bound to or that was specified in the initial socket(2) call is always used.
RFC 1191 for path MTU discovery.
RFC 791 and the <linux/ip.h> include file for the IP protocol.